GPG

GNU Privacy Guard

A well known cli PKI tool for Encryption and Digital signatures. It follows the OpenPGP standard which is the most widely used email encryption standard in the world. To get started you need a keypair:

$ gpg --gen-key

You will widely publish your public key once you export it, often through Keyservers. Your private key it is recommended you put on read only media and guard with your life. It is also wise to create a revocation certificate in case you ever misplace your private key.

GnuPG Official site